The present invention relates to the art of modulation systems, and more particularly to a system which communicates a periodic signal by modulating a carrier signal which is phase-locked to the periodic signal.
Amplitude modulation and similar forms of modulation such as double sideband, single sideband and vestigial sideband modulation are extensively used in radio broadcasting and other fields for communicating both the analog and digital signals. The fidelity of the signal being communicated is in large part determined by the frequency of the carrier signal upon which it is modulated.
Amplitude modulation, when used in conjunction with envelope detection, essentially involves the periodic sampling of the analog signal since the peak amplitude of each cycle of the carrier signal will "sample" the amplitude of the modulating function at that instant. Since this "sampling" is periodic, rather than continuous, however, those portions of the sampled function which occur between peaks of the carrier signal will be lost. If the carrier signal has a sufficiently high frequency, the modulating function will not change significantly between samples, and little modulating information will be lost. It is therefore necessary, if high fidelity is to be achieved, that the carrier signal be a high multiple (10 or greater) of the maximum frequency of the modulating signal.
Furthermore, if a periodic modulating signal (such as a timing signal) is employed, spurious variations will exist in the timing of the signal as subsequently demodulated. This is due to the "sampling" of the periodic signal at different times in different cycles. This is commonly referred to as phase jitter, and is particularly troublesome when the signal being transmitted is a timing signal since it is necessary to reduce the phase jitter of the timing signal to as small a degree as possible if precise timing is required. This again means the frequency of the carrier signal should be as high as possible.
The necessity for using high frequency carrier signals to communicate timing information can represent a significant constraint in some systems. One specific example relates to the field of television broadcasting. In order to synchronize the operation of the camera head with the network timing (commonly referred to as "house sync") it is necessary that very precise timing signals be communicated between the camera control unit and the camera head. When the camera control unit and the camera head are located quite close together, a multi-conductor cable can be provided so that each of the signals which must be communicated between the camera head and the camera control unit may be sent over a separate conductor. Thus, these timing signals may be sent to the camera head over a separate conductor. When a camera head is to be operated at a location which is quite distant from the camera control unit, however, it is desirable, both from a cost standpoint and ease of operation standpoint, that all signals be communicated over single length of flexible triaxial cable.
When only a single triax cable is used, the video signals are communicated between the camera head and the camera control unit in separate frequency channels. In view of the fact that as many as four video signals must be communicated over this cable, the frequency spectrum of the composite signal is quite broad. If an additional high frequency channel is allocated to the communication of one or more timing signals, then this spectrum must be extended even further. It is desirable, however, that the frequency spectrum be as limited as possible because of the nature of the attenuation characteristics of the triax cable. In view of this, it would be desirable to communicate the timing signal in a low frequency channel, since these channels cannot be used for the transmission of video signals due to the poor fidelity which would result therefrom. Due to the low frequency nature of the carrier signal, however, unacceptable phase jitter would accompany the transmission of the timing signals over these channels, if an envelope detector were used to recover the timing signals.
This problem could be largely avoided by using a synchronous detector, rather than an envelope detector, to recover the timing signals. This is because a synchronous detector is capable of recovering information from all parts of the modulated signal, and thus does not suffer from the sampling effects described previously. For many reasons (including cost and complexity), however, it would be desirable to avoid the use of a synchronous detector.